Realizing the full potential of otoacoustic emissions (OAEs) as noninvasive probes of cochlear function requires first understanding the physical and physiological mechanisms that generate and shape these sounds. Despite considerable recent progress identifying fundamental differences among OAEs types, many aspects of OAE generation remain poorly understood. This project focuses on a cluster of tightly interrelated issues central to the understanding of OAEs and their relation to other measures of cochlear and middle-ear function, including basilar-membrane (BM) motion, auditory-nerve tuning, and middle-ear transmission. Our approach involves a strong continuing interaction between theoretical modeling and quantitative experiments that test model predictions. The work proposed in Aim 1 will test and extend our understanding of cochlear mechanics and mechanisms of OAE generation and mixing. Specifically, we will (a) test models of reflection-source OAE generation, with a special focus on the apex of the cochlea, where preliminary work finds an unexpected discrepancy between theory and experiment; (b) determine how measurements of stimulus-frequency OAEs (SFOAEs) depend on the stimulus and analysis paradigms used to measure them; (c) explore the role of wave interference and coherence effects in the production of distortion-product OAEs (DPOAEs) by comparing the ear-canal and intracochlear "tuning" of acoustical and mechanical distortion products; and (d) test OAE-model predictions and determine the effect of multiple internal reflection of OAEs on BM motion using simultaneous measurements of OAE and BM responses to acoustic stimuli. In Aim 2 we will characterize middle-ear input/ output characteristics to understand middle-ear effects on OAEs while developing a promising noninvasive method for measuring the frequency dependence of middle-ear transmission. In Aim 3 we will test and extend our methods for using OAEs to estimate cochlear tuning and other properties noninvasively by comparing otoacoustic and auditory-nerve-fiber measurements of tuning bandwidths and group delays as a function of stimulus intensity and characteristic frequency. [unreadable] [unreadable]